U.S. patent application number 10/550468 was filed with the patent office on 2007-04-26 for powder coating and process for the preparation of thin layers in the manufacture of printed circuit boards.
This patent application is currently assigned to Atotech Deutschland GMBH. Invention is credited to Sandro Cicchetti, Jurgen Kress, Christoph Rickert.
Application Number | 20070093620 10/550468 |
Document ID | / |
Family ID | 33099289 |
Filed Date | 2007-04-26 |
United States Patent
Application |
20070093620 |
Kind Code |
A1 |
Rickert; Christoph ; et
al. |
April 26, 2007 |
Powder coating and process for the preparation of thin layers in
the manufacture of printed circuit boards
Abstract
The invention relates to a powder coating, an aqueous dispersion
based on the powder coating, a process for its preparation and a
process for the preparation of coating layers on substrates, inter
alia for the preparation of multilayer structures. The process does
not require the use of any organic solvents.
Inventors: |
Rickert; Christoph;
(Reinach, CH) ; Kress; Jurgen; (Basel, CH)
; Cicchetti; Sandro; (Magden, CH) |
Correspondence
Address: |
MCDONNELL BOEHNEN HULBERT & BERGHOFF LLP
300 S. WACKER DRIVE
32ND FLOOR
CHICAGO
IL
60606
US
|
Assignee: |
Atotech Deutschland GMBH
Berlin
DE
|
Family ID: |
33099289 |
Appl. No.: |
10/550468 |
Filed: |
March 22, 2004 |
PCT Filed: |
March 22, 2004 |
PCT NO: |
PCT/EP04/03001 |
371 Date: |
August 18, 2006 |
Current U.S.
Class: |
525/523 ;
525/529; 525/934 |
Current CPC
Class: |
H05K 3/4673 20130101;
H05K 3/4652 20130101; H05K 3/4655 20130101; H05K 2203/066 20130101;
H05K 2203/0156 20130101; H05K 2203/1355 20130101; C09D 5/03
20130101; H05K 2201/0358 20130101; C09D 163/00 20130101 |
Class at
Publication: |
525/523 ;
525/529; 525/934 |
International
Class: |
C08L 63/04 20060101
C08L063/04 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 26, 2003 |
DE |
103 13 555.3 |
Mar 26, 2003 |
DE |
103 13 556.1 |
Claims
1. Curable powder coating having a glass transition temperature of
at least 150.degree. C. in the cured state obtainable by (i) mixing
(a) a polymeric binder and at least one of an oxazine resin, a
cyanate ester or a maleimide, (b) a hardener or initiator, (c) a
coating additive, (d) optionally a filler, (e) optionally a
compatibilizing polymer and optionally further components (ii) melt
extruding the mixture obtained in step (i) and (iii) milling and
sieving the extruded mixture.
2. Powder coating according to claim 1, wherein the powder coating
has a glass transition temperature in the uncured state of at least
20.degree. C., and has a glass transition temperature in the cured
state of at least 160.degree. C.
3. Powder coating according to claim 1, wherein the polymeric
binder is a solid epoxy resin.
4. Powder coating according to claim 1 wherein the component (a)
comprises a mixture of epoxy resins with a glass transition
temperature of at least 20.degree. C.
5. Powder coating according to claim 1, wherein the epoxy resin is
selected from the group consisting of standard solid epoxy resins
based on bisphenol A and bisphenol A diglycidyl ether.
6. Powder coating according to claim 5, wherein the epoxy
equivalent weight of the epoxy resin is >300 g/equivalent.
7. Powder coating according to claim 1, wherein the epoxy resin
contains a multifunctional epoxy resin or a mixture of
multifunctional epoxy resins.
8. Powder coating according to claim 7, wherein the multifunctional
epoxy resin is selected from the group consisting of cresol-novolak
epoxy resins, phenol-novolak epoxy resins and naphthol-containing
multifunctional epoxy resins.
9. Powder coating according to claim 1, wherein the cyanate ester
is selected from the group consisting of bifunctional and
multifunctional cyanate esters.
10. Powder coating according to claim 1, wherein the maleimide is
selected from the group consisting of bifunctional and
multifunctional maleimides and the oxazine resin is selected from
the group consisting of bifunctional and multifunctional oxazine
resins.
11. Powder coating according to claim 1, wherein the hardener is
selected from the group consisting of phenolic hardeners, bisphenol
A, dicyandiamide or modified dicyandiamide, acid anhydrides,
aromatic and aliphatic amines and ring-substituted diamines.
12. Powder coating according to claim 11, wherein the hardener is
dicyandiamide or a modified dicyanamide.
13. Powder coating according to claim 1 wherein the hardener or
initiator is in an amount of 0.1 to 10 wt.-%.
14. Powder coating according to claim 1, wherein the coating
additives are in an amount of 0.1 to 10 wt.-%.
15. Powder coating according to claim 1, wherein the filler is in
an amount of 5 to 300 wt. %, based on components (a), (b) and
(c).
16. Powder coating according to claim 1 wherein the filler is an
inorganic filler.
17. Powder coating according to claim 16, wherein the filler is
fused silica or kaoline.
18. Powder coating according to claim 1 wherein the filler has an
average particle size of less than 30 .mu.m.
19. Powder coating according to claim 1 wherein the filler is an
organic filler which does not melt upon processing of the powder
coating.
20. Powder coating according to claim 1 wherein the filler is an
organic filler which melts upon processing of the powder coating
and shows phase separation upon cooling.
21. Powder coating according to claim 1 wherein the filler is
polyphenyl ether or a fluorinated thermoplastic.
22. Powder coating according to claim 1, wherein the powder
coating's coefficient of thermal expansion in the hardened state is
<70 ppm/.degree. C., in the x-, y- and z-direction.
23. Powder coating according to claim 1, wherein the powder
coating's dielectric constant in the hardened state is <3.8.
24. Powder coating according to claim 1, wherein the powder coating
is stable in storage, and wherein its exotherm does not decrease by
more than 10% upon storage for three months at 25.degree. C.
25. Powder coating according to claim 1, comprising as component
(a) about 50-90 wt.-% of epoxide and about 5-20 wt.-% of cyanate
ester, as component (b) about 0.5-5 wt.% dicyandiamide and about
0.1-2 wt.-% of 2-phenylimidazole.
26. Process for the preparation of the curable powder coating
according to claim 1, comprising the following steps: (i) mixing of
components (a), (b), (c) and optionally (d) and (e), (ii) melt
extrusion of the mixture obtained in step (i) and (iii) milling and
sieving of the extruded mixture.
27. Process according to claim 26, wherein two or more of the
components (a), (b), (c), (d) and (e) are used as a master batch in
step (i).
28. Process according to claim 26 wherein step (ii) is carried out
such that the conversion of the reactive component is less than
20%.
29. Process for the preparation of coating layers on substrates
comprising the following steps: (i) wet milling of the powder
coating according to claim 1, optionally with further additives to
prepare a dispersion, (ii) applying the dispersion to the substrate
and (iii) heat treating the coated substrate.
30. Process according to claim 29, wherein the heat treatment in
step (iii) is carried out such that, after applying the dispersion
to the substrate, the film is first dried and melted and
subsequently cured.
31. Process according to claim 29, wherein the heat treatment of
the coated substrate in step (iii) is carried out such that, after
applying the dispersion to the substrate, a single step of drying,
melting and curing the powder coating is carried out.
32. Process for preparing a multilayer structure comprising the
following steps: (i) wet milling of the powder coating according to
claim 1, optionally with further additives to prepare a dispersion,
(ii) applying the dispersion to a structured substrate, (iii) heat
treating the coated substrate, (iv) drilling and metallizing, (v)
optionally repeating steps (ii) and (iv).
33. Process according to claim 29 wherein the substrate is a copper
sheet, a polymeric support sheet, a structured printed circuit
board or a core layer thereof.
34. Process according to claim 33, wherein the support sheet is
combined with woven or non-woven fabric of glass fibre or aramide
fibre.
35. Process according to claim 29 wherein antifoaming agents,
wetting agents, biocides, rheologic additives or flow-control
agents are used as additives.
36. Process according to claim 29 wherein the heat treatment or the
curing is effected by (a) melting in an oven with or without
convection, (b) infrared radiation, (c) near infrared radiation
(NIR), (d) induction or (e) excitation by microwaves.
37. Process for preparing coating layers on substrates comprising
the following steps: (i) applying the powder coating according to
claim 1 to a substrate, (ii) melting the powder coating and (iii)
curing the powder coating.
38. Process for preparing a multilayer structure comprising the
following steps: (i) applying the powder coating according to claim
1 to the substrate, (ii) melting the powder coating followed by
cooling, (iii) laminating the coated substrate to a printed circuit
board which may already comprise more than one layer, (iv) curing,
(v) drilling and through-connecting the individual layers and
substrates to prepare a multilayer structure, (vi) optionally
repeating steps (i) to (v).
39. Process according to claim 37 wherein the substrate is a copper
sheet or a polymeric support sheet.
40. Process according to claim 39, wherein the support sheets are
combined with woven or non-woven fabric of glass fibre or aramide
fibre.
41. Process for the preparation of a multilayer structure
comprising the following steps: (i) applying the powder coating
according to claim 1 to a structured substrate, (ii) melting and
curing the powder coating layer followed by cooling, (iii)
drilling, (iv) metallizing, (v) optionally repeating steps (i) to
(iv).
42. Process according to claim 37 wherein the application of the
powder coating is effected by spraying, electromagnetic brush
coating, powder cloud coating or roller coating.
43. Process according to claim 42, wherein the spraying is effected
by coronar charging or triboelectric charging.
44. Process according to claim 37 wherein the melting is effected
by (a) melting in an oven with or without convection, (b) infrared
radiation, (c) near infrared radiation (NIR), (d) induction or (e)
excitation by microwaves.
45. Process according to claim 32, wherein the substrate is a
copper sheet, a polymeric support sheet, a structured printed
circuit board or a core layer thereof.
46. Process according to claim 45, wherein the support sheet is
combined with woven or non-woven fabric of glass fibre or aramide
fibre.
47. Process according to claim 32, wherein antifoaming agents,
wetting agents, biocides, rheologic additives or flow-control
agents are used as additives.
48. Process according to claim 32, wherein the heat treatment or
the curing is effected by (a) melting in an oven with or without
convection, (b) infrared radiation, (c) near infrared radiation
(NIR), (d) induction or (e) excitation by microwaves.
49. Process according to claim 38, wherein the substrate is a
copper sheet or a polymeric support sheet.
50. Process according to claim 49, wherein the support sheets are
combined with woven or non-woven fabric of glass fibre or aramide
fibre.
51. Process according to claim 41, wherein the application of the
powder coating is effected by spraying, electromagnetic brush
coating, powder cloud coating or roller coating.
52. Process according to claim 51, wherein the spraying is effected
by coronar charging or triboelectric charging.
53. Process according to claim 41, wherein the melting is effected
by (a) melting in an oven with or without convection, (b) infrared
radiation, (c) near infrared radiation (NIR), (d) induction or (e)
excitation by microwaves.
Description
[0001] The invention relates to a powder coating, a process for its
preparation and a process for the preparation of coating layers on
substrates, in particular, on printed circuit boards.
[0002] The preparation of dielectric layers is known per se and is
essentially carried out by using dry films, i.e., sheets consisting
of several layers.
[0003] One of these layers is the not yet fully cured, still
reactive resin (B-stage). This is stabilized by a support layer
(for example, copper, PET) and is covered on the other side by a
protective layer (for example, of PE). The application is carried
out in such a way that the protective layer is removed and the
remaining sheet is laminated onto a structured printed circuit
board. In the case where a PET support layer is used, the support
layer of PET is peeled of after heat curing. In variants of this
process, a further resin layer, which is already fully cured
(C-stage), is present between the reactive resin layer (B-stage)
and the support layer. The advantage of this method lies in better
control of the minimal layer thickness of the dielectric and in
better planarity of the layer at the end of the overall
process.
[0004] These dry films are described in Charles A. Harper, High
Performance Printed Circuit Boards, 1999, McGraw-Hill, Chapter 2.
Because of the high reactivity of the resin layer, the material
must be stored and shipped at low temperatures (<0.degree. C.)
which causes extra costs and requires considerable logistic
capabilities. The layer is typically prepared by applying a liquid
formulation to the support layer, i.e., the formulation must be
capable of being formulated as a liquid. Furthermore, solvent
emissions occur upon drying.
[0005] The process described above have the disadvantage that
certain fillers can be incorporated only with difficulty or not at
all. As a general rule, the filler must be capable of being
dispersed stably in an organic solvent.
[0006] A further disadvantage is the low storage stability of the
aforementioned dry films and the necessity to store and ship these
at low temperatures.
[0007] The object of the invention is to provide a powder coating,
a dispersion on the basis thereof, a process for the preparation of
the coating or the dispersion and a process for preparing thin
coating layers on substrates, in particular, on copper sheet for
the preparation of printed circuit boards, which do not have these
disadvantages.
[0008] All conceivable fillers are to be useable in the powder
coating and in the process of the invention.
[0009] Furthermore, the use of organic solvents is to be
avoided.
[0010] Moreover, the use of the powder coating or the process of
the invention is to make it possible to prepare thin dielectric
coating layers with improved properties on structured or
non-structured substrates.
[0011] The invention provides a curable powder coating which is
obtainable by
[0012] (i) mixing [0013] (a) a polymeric binder, an oxazine resin,
a cyanate ester or a maleimide, [0014] (b) a hardener or initiator,
[0015] (c) a coating additive, [0016] (d) optionally a filler,
[0017] (e) optionally a compatibilizing polymer [0018] and
optionally further components
[0019] (ii) melt extruding the mixture obtained in step (i) and
[0020] (iii) milling and sieving the extruded mixture.
[0021] According to a preferred embodiment of the invention, the
powder coating has a glass transition temperature in the uncured
state of at least 20.degree. C., preferably at least 25.degree. C.
and more preferably at least 30.degree. C. and has a glass
transition temperature in the cured state of at least 150.degree.
C., preferably at least 160.degree. C. and more preferably at least
170.degree. C.
[0022] Furthermore, the polymeric binder is preferably essentially
an epoxy resin which is solid at room temperature. The glass
transition temperature of the resin should preferably be at least
25.degree. C.
[0023] The powder coating of the invention can preferably also
comprise a mixture of epoxy resins. This mixture preferably has a
glass transition temperature of >25.degree. C. in the uncured
state. Its molecular weight (number average molecular weight) is
generally >600.
[0024] Suitable epoxy resins for the preparation of the powder
coating of the invention are described, for example, in: Clayton A.
May (Ed.) Epoxy Resins: Chemistry and Technology, 2nd ed., Marcel
Dekker Inc., New York, 1988.
[0025] Preferred mixtures of epoxy resins on the basis of bisphenol
A and bisphenol A diglycidyl ether. The epoxy equivalent weight of
these resins is >300 g/equivalent. Such a resin is, for example,
D.E.R. 6508 (available from Dow Chemicals).
[0026] Epoxy resins on the basis of bisphenol F and bisphenol S can
optionally also be added.
[0027] Furthermore, the mixture can comprise multifunctional epoxy
resins. The functionality of these resins is >3. Examples for
such multifunctional epoxy resins are cresol-novolak epoxy,
phenol-novolak epoxy and naphthol-containing multifunctional epoxy
resins.
[0028] Examples for the aforementioned epoxy resins are bisphenol A
epoxy resin such as D.E.R. 667-20, D.E.R. 663UE, D.E.R. 692H,
D.E.R. 692, D.E.R. 662E, D.E.R. 6508, D.E.R. 642U-20 (available
from Dow Chemicals), cresol-novolak epoxy resins such as Araldite
ECN 1299, Araldite ECN 1280 (Vantico), EOCN-103 S, EOCN-104,
NC-3000, EPPN 201, EPPN-502 H (Nippon Kayaku), naphthol epoxy
resins such as NC 7000-L (Nippon Kayaku) and brominated Epoxy
resins such as Araldite 8010 (Vantico), BREN-S (Nippon Kayaku),
ESB-400 T (Sumitomo) and Epikote 5051 (Resolution). Moreover,
modified epoxy resins can also be used. Such modifications are, for
example, the use of chain reaction terminating agents to control
the molecular weight, so-called "high-flow" resins, and the use of
multi-functional monomers to prepare branched resins.
[0029] A particularly preferred powder coating of the invention
comprises, as component (a), about 50-90 wt.-% of epoxide and about
5-20 wt.-% of cyanate ester, as component (b), about 0.5-5 wt.-% of
dicyandiamide and about 0.1-2 wt.-% of 2-phenyl-imidazole, for
example about 85 wt.-% of epoxide, 10 wt.-% of cyanate ester, about
2 wt.-% of dicyandiamide as hardener and about 1 wt.-% of
2-phenylimidazole as initiator.
[0030] As mentioned above, apart from the epoxy resins, cyanate
esters can also be used as polymeric binders. In the preparation of
the powder coating of the invention, these can be used both in
monomeric form as well as in the form of oligomers or
prepolymers.
[0031] Suitable cyanate esters are bifunctional cyanate esters,
such as BADCy, Primaset Fluorocy, Primaset MethylCy, or
multifunctional cyanate esters, such as Primaset BA-200, Primaset
PT 60, Primaset CT 90, Primaset PT 30. All of the aforementioned
bifunctional and multifunctional cyanate esters are available from
Lonza, Basel, Switzerland.
[0032] Especially preferred cyanate esters are BADCy and its
prepolymers (e.g. Primaset BA-200).
[0033] Apart form the cyanate esters, the component (a) can also
comprise 1-oxa-3-aza-tetralin-containing compounds (oxazine
resins). In the preparation of the powder coating of the invention,
these are also initially employed in monomeric form.
[0034] Preferred oxazine resins are those which are obtained either
by reacting bisphenol A with aniline and formaldehyde or by
reacting 4,4'-diaminodiphenyl methane with phenol and formaldehyde.
Further examples may be found in WO 02/072655 and EP 0 493 310 A1
as well as in WO 02/055603 and the Japanese patent applications JP
2001-48536, JP 2000-358678, JP 2000-255897, JP 2000-231515, JP
2000-123496, JP 1999-373382, JP 1999-310113 and JP 1999-307512.
Further examples may be found in Macromolecular Chemistry,
Macromolecular Symposia (1993), 74 (4th Meeting on Fire Retardant
Polymers, 1992), 165-71, EP 0 493 310 A1, EP 0 458 740 A1, EP 0 458
739 A2, EP 0 356 379 A1 and EP 0 178 414 A1.
[0035] The maleimides used in the preparation of the powder coating
of the invention are also known per se to the skilled person and
are described, for example, in Shiow-Ching Lin, Eli M. Pearce,
High-Performance Thermosets, Carl Hanser Verlag, Munich 1994,
Chapter 2.
[0036] The component (b) of the resin composition of the invention
comprises a hardener or initiator. Such hardeners and initiators
are known per se to the skilled person and comprise latent
hardeners with low activity at room temperature, such as phenolic
hardeners, such as D.E.H. 90, D.E.H. 87, D.E.H. 85, D.E.H. 84,
D.E.H. 82 (available from Dow Chemicals, US), dicyandiamide or
derivatives thereof, such as Dyhard OTB, Dyhard UR 200, Dyhard UR
300, Dyhard UR 500, Dygard 100, Dyhard 100 S, Dyhard 100 SF and
Dyhard 100 SH (available from Degussa, Germany), bisphenol A, acid
anhydrides, such as phthalic acid anhydride, tetrahydrophthalic
acid anhydride, trimellitic acid anhydride, pyromellitic acid
anhydride, hexahydrophthalic acid anhydride, HET-acid anhydride,
dodecenyl succinic acid anhydride,
bicyclo[2.2.1]hept-5-en-2,3-dicarboxylic acid anhydride, aromatic
and aliphatic amines, such as diaminodiphenylsulfone,
diaminodiphenylether, diaminodiphenylmethane or ring-substituted
dianilines, such as Lonzacure.RTM. M-DEA, Lonzacure.RTM. M-DIPA,
Lonzacure.RTM. M-MIPA, Lonzacure.RTM. DETDA 80 (all of the
aforementioned compounds are available from Lonza, Basel,
Switzerland).
[0037] Preferably, dicyandiamide or modified dicyandiamide is
employed.
[0038] In the resin composition of the invention, the hardeners or
initiators are used in an amount of below 10 wt.-%, preferably
below 5 wt.-% (lower limit: about 0.1 wt.-%).
[0039] Preferred initiators are imidazoles and derivatives thereof,
such as 2-methylimidazole, 2-ethyl-4-methylimidazole,
2-phenylimidazole, 2-phenyl-4-methylimidazole,
bis(2-ethyl-4-methylimidazole), 2-undecylimidazole,
2,4-diamino-6(2'-methyl-imidazole(1'))ethyl-s-triazine and
1-cyanoethyl-2-undecylimidazole. Furthermore salts formed from
imidazoles and carboxylic acids can be used. Further initiators are
1,8-diaza-bicyclo(5.4.0)undecene (DBU) and boron-trihalide-amine
complexes, such as BF3-amine. Further examples may be found in
Clayton A. May (Ed.) Epoxy Resins: Chemistry and Technology, 2nd
ed., Marcel Dekker Inc., New York, 1988.
[0040] The resin composition of the invention further comprises
coating additives as component (c). These comprise flow-control
agents, degassing agents and lubricants. These are known per se to
the skilled person. Typical examples are butyl acrylate polymers as
flow-control agents, benzoin as degassing agents and waxes as
lubricants. Furthermore, for example stabilizers can be used as
coating additives.
[0041] The resin composition of the invention contains the coating
additives in an amount of generally 0.1-10 wt.-%, preferably 0.2-5
wt.-%.
[0042] Coating additives also comprise adhesion promoters. These
are useful for providing adhesion to the copper substrate.
[0043] The powder coating of the invention may further comprise
organic and inorganic fillers (d).
[0044] These fillers are suitably employed in the powder coating of
the invention in an amount of 5 to 300 wt.-%, preferably 10 to 200
wt.-%, more preferably 10 to 100 wt.-%. The stated amounts relate
to the sum of components (a), (b) and (c) of the powder
coating.
[0045] Examples for organic fillers are fluorine containing
polymers, such as polytetra-fluoroethylene (PTFE),
tetrafluoroethylene/hexafluoropropylene copolymer (FEP),
tetrafluoroethylene/ethylene copolymer (E/TFE),
tetrafluoroethylene/hexafluoro-propylene/vinylidene fluoride
terpolymer (THV), poly(trifluorochloroethylene) (PCTFE),
trifluorochloroethylene/ethylene copolymer (E/CTFE), poly(vinyl
fluoride) (PVF), poly(vinylidene fluoride) (PVDF), perfluoroalkoxy
copolymer (PFA), tetra-fluoroethylene/perfluoromethylvinylether
copolymer (MFA), furthermore poly(vinyl chloride) (PVC), polyphenyl
ether (PPO), polysulfone (PSU), polyaryl ether sulfon (PES),
polyphenyl ether sulfon (PPSU), polyphenylene sulfide (PPS),
polyether ketone (PEK) and polyether imide (PEI).
[0046] Especially preferred organic fillers are
tetrafluoroethylene/hexafluoropropylene copolymer (FEP),
ethylenetetrafluoroethylene copolymer (ETFE) and polyphenyl ether
(PPO).
[0047] In the powder coating of the invention, there may preferably
be used organic fillers which do not melt upon processing.
Alternatively, there can be used fillers which melt and show phase
separation upon cooling.
[0048] Apart from the organic fillers, inorganic fillers may also
be used in the powder coating of the invention.
[0049] Such fillers are, for example, fused silica, such as Silbond
800 EST, Silbond 800 AST, Silbond 800 TST, Silbond 800 VST, Silbond
600 EST, Silbond 600 AST, Silbond 600 TST, Silbond 600 VST
(available from Quarzwerke Frechen, Germany), fumed silica, such as
Aerosil 300 and Aerosil R 972, precipitated silica, such as
Ultrasil 360, Sipernat D 10, Sipernat 320 (available from Degussa,
Germany), calcined kaoline, such as PoleStar (Imerys, St Austell,
UK), Santintone (Engelhard Corporation, Iselin, N.J., US),
aluminium oxide, magnesium oxide, zirconium oxide, aluminium
silicates, calcium carbonate and barium sulfate, silica glass and
kaoline being preferred fillers. Furthermore, there may be
mentioned ceramics; especially those with low or negative
coefficients of expansion.
[0050] The advantages of the powder coating of the invention are
that it is possible, in order to optimise the properties of the
product, to select from a variety of fillers the one which best
satisfies the relevant requirements. For example, a given epoxy
resin mixture can, thus, be modified as needed. Even fillers which
are difficult to process can be incorporated without problems.
Thus, electric properties such as the dielectric constant
(D.sub.k), the dielectric loss factor (tan .delta.), the breakdown
resistance, the surface resistance, the volume resistance and
mechanical properties such as bending strength, impact strength,
tensile strength as well as further material properties such as the
coefficient of thermal expansion (CTE), flammability and others can
be adapted as desired. The filler does not have to be solvable or
stably dispersable in organic solvents. Consequently, it is
possible to use materials as fillers which could previously not or
only hardly be used in sequential build-up (SBU), such as the
aforementioned organic fillers.
[0051] The electrical and mechanical properties of the powder
coating and of the coating layer prepared therefrom can be
influenced and controlled by the fillers.
[0052] Thus, for example fillers with a low dielectric constant,
such as PTFE, FEP and kaoline may be employed in order to prepare
coating layers with a correspondingly low dielectric constant.
[0053] Further electrical properties can be controlled in an
analogous way.
[0054] The mechanical properties which can be influenced by the
fillers comprise, in particular, properties such as the coefficient
of thermal expansion, impact strength, and tensile strength.
[0055] The following fillers are particularly suitable for
controlling the coefficient of thermal expansion: silica glass,
kaoline, calcium carbonate and ceramics with a negative coefficient
of expansion.
[0056] Bending strength can be influenced or controlled, for
example, by PPO.
[0057] According to a preferred embodiment of the invention, the
cured powder coating has a coefficient of thermal expansion (CTE)
of <70 ppm/.degree. C. and preferably <60 ppm/.degree. C. in
the x-, y- and z-direction.
[0058] According to a further preferred embodiment, the dielectric
constant of the coating in the cured state is <3.8, preferably
<3.6. Moreover, glass transition temperatures of the cured
formulation of above 150.degree. C., preferably above 160.degree.
C., are preferred.
[0059] Furthermore, flame-retardant materials may be used as
fillers. Examples for these are inorganic materials which release
water upon heating, such as aluminium hydroxide, which is
available, for example, as Martinal OL-104, Martinal OL-111
(Martinswerk GmbH, Bergheim, Germany) or Apyral 60 D (Nabaltec,
Schwandorf, Germany), magnesium hydroxide, available, for example,
as magnesium hydroxide 8814 (Martinswek GmbH, Bergheim, Germany) or
Mg-hydroxide SIM 2.2 (Scheruhn Industrie-Mineralien, Hof, Germany),
phosphorous-containing organic compounds, such as triphenyl
phosphate (TPP), tricresyl phosphate (TCP), cresyl diphenyl
phosphate (CDP), tertiary phosphin oxides, such as Cyagard.RTM. and
Reoflam.RTM. 410, red phosphorous in the form of a dispersion in an
epoxy resin, such as Exolit RP 650, or in the form of a powder,
such as Exolit OP 930 (both products are available from Clariant
GmbH, Frankfurt, Germany) and antimony trioxide.
[0060] Furthermore, the flammability of the powder coating of the
invention can be influenced and controlled by component (c), i.e.,
the coating additives. In this connection, for example,
phosphorous-containing and nitrogen-containing flame retardants may
be mentioned.
[0061] The powder coating of the invention can, optionally, further
contain compatibelizing polymers. Such competibilizing polymers
are, for example, di- or triblock co-polymers such as
styrene/butadiene/styrene or styrene/butadiene/methyl meth-acrylate
blockcopolymers (Atofina, France).
[0062] Furthermore, the powder coating of the invention can contain
conventional additives which are conventionally used in the
processing of epoxy resins.
[0063] In the preparation of the powder coating of the invention,
the components (a), (b), (c) and, optionally, (d) and (e) are first
dry-milled to give a powder.
[0064] In doing so, It may be useful to mix and extrude individual
components beforehand to prepare a master batch.
[0065] This procedure must be used, in particular, when certain
components are difficult to incorporate. These are then
incorporated into each other beforehand. Such master batches are
also commercially available. For example, in the case of the
resins, for example, it is possible to mix two resins beforehand.
This course of action is used, in particular, when one of the
resins has a low glass transition temperature. Moreover, this
procedure may be used when certain components are used only in
small amounts.
[0066] The aforementioned components or master batches are premixed
and milled in the dry state. Before milling, the mixture may
optionally be cooled.
[0067] After thorough mixing (and optional cooling), the material
is milled in the dry state while maintaining a powder and the
powder is subsequently extruded. This extrusion provides complete
homogenisation of the components and is a key step in the overall
process.
[0068] After extrusion, the material is milled in the dry state and
the oversize material is separated, wherein a sieve size in the
range of less than 10 to 500 .mu.m and preferably less than 100
.mu.m is suitably used, which guarantees a corresponding particle
size. Classifying mills such as Hosekawa MicroPul are particularly
suitable for milling.
[0069] The aforementioned melt extrusion is preferably carried out
in such a way that the conversion of the reactive component is less
than 20%, preferably less than 10%. This reaction is due to the
fact that a melt is formed upon extrusion. The degree of conversion
can be determined by the skilled person by thermal analysis. The
corresponding extrusion parameters (for obtaining such a degree of
conversion) can be determined by the skilled person by simple
experiments. They depend on the type of extruder and the type and
amount of the components employed. For example, a Buss co-kneader
can be used as extruder, in which the aforementioned components are
extruded. As mentioned above, the mass is subsequently cooled and
reduced to small pieces. The final powder coating mixtures
preferably have an average particle size in the range of 1 to 500
.mu.m, especially of 10 to 100 .mu.m.
[0070] The powder coating thus prepared is used according to the
invention for the preparation of coating layers on substrates which
are subsequently employed in the manufacture of printed circuit
boards.
[0071] The invention further provides a process for preparing
coating layers on substrates, comprising the following steps:
[0072] (i) applying the powder coating of the invention to a
substrate,
[0073] (ii) melting the powder coating and
[0074] (iii) curing the powder coating.
[0075] In the process of the invention, thin dielectric coating
layers, i.e., layers with a thickness of about 5 to 500 .mu.m, are
prepared. Therefore, the process of the invention may be used in
the manufacture of printed circuit boards and, in particular, in
the so-called sequential-build-up process (SBU). Other possible
uses are in the application of solder stop masks and in all other
processes in which the preparation of thin layers is required and
which are characterized in that fillers are used which are not or
only poorly soluble in common solvents under normal process
conditions.
[0076] The powder coating can be applied to the substrate by
various methods. Thus, the application of the powder coating can be
effected, for example, by spraying, electromagnetic brush coating,
powder cloud coating or roller coating.
[0077] The spraying can be carried out, for example, by coronar
charging or triboelectric charging. These processes are known to
the person skilled in the art. Triboelectric charging is preferably
used in the process of the invention.
[0078] Furthermore, in the process of the invention, the powder
coating can be applied by means of rollers. In this case, the
powder is applied to the substrate by means of a sieve and
subsequently treated with a roller. The roller can be heated.
[0079] The application by means of the electromagnetic brush
technology is described in WO 96/15199.
[0080] The powder cloud technology is described, for example, in
Proceedings--International Conference in Organic Coatings:
Waterborne, High Solids, Powder Coatings, 23rd, Athens, Jul. 7-11,
1997 (1997), 139-150 Publisher: Institute of Materials Science, New
Paltz, N. Y.; Journal fur Oberflachentechnik (1996), 36(8), 34-36,
39; Deutsche Forschungsgesellschaft fur Oberflachenbehandlung
(2000), 44 (Pulverlack-Praxis), 95-100; Journal fur
Oberflachentechnik (1998), 38(2), 14-18 and in WO 97/47400.
[0081] The following methods are, in principle, suitable for
melting the powder coating layer:
[0082] a) melting in an oven with or without convection,
[0083] b) infrared radiation,
[0084] c) near infrared (NIR) and
[0085] d) induction and optionally
[0086] e) excitation by microwaves.
[0087] In the process of the invention, the melting is preferably
be effected by NIR. This method is described in WO 99/47276, DE
10109847, in Kunststoffe (1999), 89 (6), 62-64 and in Journal fur
Oberflachentechnik (1998), 38 (2), 26-29.
[0088] The step of melting is particularly important. Upon melting,
a change in viscosity occurs, i.e., the powder first melts. The
viscosity of the melt decreases. Subsequently, curing and, thus, a
rise in viscosity takes place. This operation must be conducted in
the process of the invention in such a way that the viscosity of
the melt is initially as low as possible and subsequently good flow
is achieved without formation of bubbles, such that a non-porous
film is obtained.
[0089] An essential advantage of the process of the invention is to
be seen in the fact that the coating layer is first melted, remains
flowable and can, thus, be used for the preparation of a multilayer
structure.
[0090] The invention further provides a process of the preparation
of a multilayer structure comprising the following steps:
[0091] (i) applying the powder coating of the invention to a
substrate,
[0092] (ii) melting the powder coating followed by cooling,
[0093] (iii) laminating the coated substrate to a printed circuit
board which may already comprise more than one layer,
[0094] (iv) curing,
[0095] (v) drilling and through-connecting the individual layers
and substrates to prepare a multilayer structure,
[0096] (vi) optionally repeating steps (i) to (v).
[0097] In this process, the powder coating is preferably applied by
the aforementioned electromagnetic brush technology (EMB). In this
way, a more homogeneous application of the powder and, thus, a more
homogeneous layer thickness may be achieved. The melting is
preferably effected by the NIR method. In this way, pore-free
coating layers are obtained.
[0098] It is an important feature of this process that the curing
takes place only in step (iv), i.e., after formation of the
multilayer structure. In this connection, it is important that the
films are still flowable during the preparation of the
structure.
[0099] The curing of the melted powder coated layers takes place
during pressing or lamination. The pressing or lamination takes
place under vacuum and pressure, the corresponding parameters being
known to the skilled person. For example, a Lauffer press or an
Adara press can be used. The pressing cycles are to be adapted to
the individual material used.
[0100] In the last step of this process, the press contacting of
the individual layers and substrates takes place in order to
prepare the multilayer structure.
[0101] Typical substrates are, in particular, copper sheets or
polymeric support sheets. These may further be combined with woven
or non-woven fabrics of glass fibre or aramide fibre.
[0102] When structured substrates are used, the process of the
invention comprises the following steps:
[0103] (i) applying the powder coating of the invention to the
structured substrate,
[0104] (ii) melting and curing the powder coating layer followed by
cooling,
[0105] (iii) drilling,
[0106] (iv) metallizing,
[0107] (v) optionally repeating steps (i) to (iv).
[0108] In this process, the powder coating is preferably applied by
the aforementioned electromagnetic brush technology (EMB). In this
way, a more homogeneous application of the application and, thus, a
more homogeneous layer thickness and better edge coverage can be
achieved.
[0109] The invention further provides a process for the preparation
of coating layers on substrates comprising the following steps:
[0110] (i) wet milling the powder coating of the invention,
optionally with further additives to prepare a dispersion,
[0111] (ii) applying the dispersion to the substrate and
[0112] (iii) heat treating the coated substrate.
[0113] In the first step of the process of the invention, a
dispersion is prepared from the powder coating by addition of
water. The solids content of the dispersion is generally 20 to 70
wt.-%, preferably 30 to 60 wt.-%.
[0114] In order to prepare the dispersion, the powder is milled
with water, optionally adding further additives. In addition to the
aforementioned additives, which may be employed in an amount of 0.1
to 5%, preferably 0.5 to 2.5 wt.-%, wetting agents, dispersants,
antifoaming agents and degassing agents as well as flow-control
agents may be added.
[0115] Examples for such wetting agents and dispersants are
solutions of high molecular weight block copolymers containing
groups with pigment affinity, such as Disperbyk 160, 170 or 182,
acrylate copolymers containing groups with pigment affinity, such
as Disperbyk 116, solutions of alkylammonium salts, such as
Disperbyk 140, solutions of salts of unsaturated polyamino amides
and of acidic or polar esters, such as Anti Terra U or Disperbyk
101 (all from Byk Chemie, Wesel, Germany), polycarboxylic acid
polymers with or without polysiloxane copolymer such as Byk P 104
or Byk 220 S, fluorine-containing wetting agents such as Zonyl FSN
or Zonyl FSH (both from DuPont) and non-ionic surfactants such as
products of the Surfynol series from Air Product, Utrecht, NL.
[0116] Examples for antifoaming agents and degassing agents are
silicone-free foam-destroying polymers, such as Byk 051, solutions
or emulsions of foam-destroying polysiloxanes, such as Byk 020 or
Byk 067, silicone-free foam-destroying polymers and hydrophobic
solids, such as Byk 011, emulsions and mixtures of paraffin base
mineral oils and hydrophobic components, such as Byk 033 or Byk 036
(all from Byk Chemie, Wesel, Germany).
[0117] Examples for flow-control agents are polyether-modified
polydimethyl siloxanes, such as Byk 300 or Byk 085, modified,
hydroxyfunctional polydimethylsiloxanes, such as Byk 370,
polyether-modified polydimethylsiloxanes, such as Byk 345 and
ionogenic and non-ionogenic polyacrylate copolymers, such as Byk
380.
[0118] Further examples for additives may be found in WO 96/32452,
WO 96/37561, WO 97/01609, WO 97/17390, WO 99/15593, EP 0 714 958 A2
and EP 0 044 810 A1.
[0119] The process of the invention has the advantage that the
fillers are located in the coating particles and, therefore, no
demixing occurs because the filler has been incorporated into the
coating particles, which is not soluble in water. This is a
particular advantage of the process of the invention: Upon
processing the solvent containing formulations known in the prior
art, either the filler sediments or must be stabilized by talking
special measures, i.e., different filler concentrations occur
within the formulations.
[0120] In the process of the invention, insoluble coating particles
are used which contain the filler homogeneously dispersed therein
and, therefore, no concentration differences occur.
[0121] In order to be able to obtain a stable dispersion, the
particles must have an average size which is smaller than 10 .mu.m,
preferably smaller than 7 .mu.m. The particle size can be
determined by means of a Coulter counter.
[0122] After applying the dispersion to the substrate, a heat
treatment takes place which serves to remove the dispersion medium
and to melt the powder coating layer. The heat treatment can be
carried out such that, after applying the dispersion to the
substrate, the film is first dried and melted and subsequently
cured. Alternatively, only a single step of drying, melting and
curing the powder coating can be carried out after applying the
dispersion to the substrate.
[0123] The aforementioned methods are suitable for the heat
treatment and, in particular, the melting of the coating layer.
[0124] Furthermore, the invention provides a process for preparing
a multilayer structure, comprising the following steps: [0125] (i)
wet milling the powder coating of the invention, optionally with
further additives to prepare a dispersion, [0126] (ii) applying the
dispersion to a structured substrate, [0127] (iii) heat treating
the coated substrate, [0128] (iv) drilling, metallizing and
structuring, [0129] (v) optionally repeating the steps (ii) to
(iv).
[0130] In summary, it may be noted that the powder coating of the
invention and the process of the invention provide a possibility to
prepare coating layers on substrates, in particular, printed
circuit boards, without requiring any organic solvent.
[0131] The absence of any organic solvent is an important aspect in
view of industrial safety and the resulting air venting systems,
waste disposal, environmental protection requirements becoming
constantly stricter and the costs associated with these
factors.
[0132] An important advantage of the described process in
industrial practice is the fact that the material to be applied is
a one-component system, i.e., the binder (epoxy resin) and the
hardener are already present in the actual composition and do not
have to be mixed immediately before application.
[0133] A further advantage compared to dry films is the storage
stability at normal temperatures of transport and storage. The term
"storage stable" refers to a resin composition whose components do
not react and, in particular, a composition whose exotherm does not
decrease by more than 10% over a period of about three months (upon
storage at 25.degree. C.).
[0134] The invention is illustrated in more detail by the following
examples.
EXAMPLE 1
[0135] 4444 g D. E. R. 6508 (Dow Chemicals) were melted in an oven
at 110.degree. C. After addition of 1460 g melted Primaset BA-200
(Lonza) and 74 g bisphenol A (Aldrich), 60 g Modarez and 12 g
benzoin (Aldrich), the mixture was mixed thoroughly. After cooling
with liquid nitrogen, the material was milled in a dry state and
extruded (twin-screw extruder EBVP from OMC: 110-120.degree. C.,
500 rpm).
[0136] The powder had a glass transition temperature of 45.degree.
C. (DSC).
[0137] After repeated dry milling and sieving (100 .mu.m), the
powder was applied to a copper sheet in a thickness of 32 .mu.m by
means of an EMB machine (Epping). After melting at 160.degree. C.
over 5 min, the powder showed excellent flow. The film is free of
bubbles.
[0138] After thermal curing at 190.degree. C. for 20 minutes, the
layer thickness of the dielectric was determined to be 45 .mu.m.
The T.sub.g (DSC) was 172.degree. C.
EXAMPLE 2
[0139] 290 g DER 6508, 58 g NC 7000-L, 58 g BADCy, 11.7 g DICY, 180
g Silbond EST 800 and 0.6 g phenylimidazole were mixed in a
premixer and subsequenetly extruded (twin-screw extruder EBVP from
OMC: 110-120.degree. C., 500 rpm). After dry milling and sieving, a
part of the powder was cured and the coefficient of expansion was
measured by TMA (CTE=55 ppm, Dk (at 1 GHz)=3.6).
EXAMPLE 3
[0140] 350 g DER 6508, 70 g NC 7000-L, 202 g Silbond EST 800 and 50
g ETFE ET 6235 were thoroughly mixed in a premixer and extruded at
260.degree. C. The material was milled and sieved and mixed with 70
g Primaset BA-200, 14 g DICY, 0.7 g phenyimidazole, 1.5 g benzoin
and 11.4 g Modarez and then extruded at 130.degree. C. (twin-screw
extruder EBVP from OMC: 110-120.degree. C., 500 rpm). After milling
and sieving, the powder was applied to a copper sheet by means of
an EMB machine (Epping) and cured at 190.degree. C. Dk (at 1
GHz)=3.3.
EXAMPLE 4
[0141] The powder of Example 1, after dry milling, was sieved
through a 100 .mu.m sieve. 250 g of this powder were stirred and
mixed with 374 g demineralised water, 0.37 g Surfynol 440, 2.77 g
Disperbyk 185 and 4.16 g antifoaming agent Byk 028. This dispersion
was milled twice in a DynoMill (Bachofen, Basel, CH) (grinding
material ZrO.sub.2, diameter 0.8 mm; gap width 0.3 mm; motor speed
10 m/s). Then 124 g powder, 0.185 g Surfynol 440, 0.277 g Disperbyk
185, 0.369 g Byk 028 and 0.28 g Aerosil R 972 were again added and
the mixture was again milled twice as described above. The highly
viscous material was slowly stirred over night in order to allow
the obtained foam to disappear. The suspension thus obtained (which
had become less viscous) was again mixed with 0.28 g Aerosil and
then applied to a copper-clad FR-4 laminate by means of a doctor
knife and cured in an oven at 190.degree. C. The layer thickness
was determined by means of a layer thickness measuring device
(Isoscope, Fischer) to be 45 .mu.m. The coating layer was smooth
and pore-free and had a glass transition temperature (Tg) of
165.degree. C.
COMPARATIVE EXAMPLE 1
[0142] 43.7 g D. E. R. 6508 (Dow Chemicals) was dissolved with 14.4
g Primaset BA-200 (Lonza), 0.73 g bisphenol A (Aldrich), 0.59 g
Modarez and 0.12 g benzoin in 150 ml MPA. After addition of 5.5 g
poly(tetrafluoroethylene) (Ausimont) a white solid floats on the
surface so that this mixture cannot be applied.
COMPARATIVE EXAMPLE 2
[0143] The powder of Example 3 was suspended in MPA. The material
formed a viscous mass which deposited at the bottom of the vessel
and could not be applied to a copper sheet.
* * * * *